Add py.typed for type hints

[UFO-101]

Description

I think that all libraries are supposed to include a py.typed file for types to be accessible to external users. See: https://blog.whtsky.me/tech/2021/dont-forget-py.typed-for-your-typed-python-package/.

However, I haven't researched this thoroughly, so someone should probably look into this a bit more.

Type of change

• New feature (non-breaking change which adds functionality)

Checklist:

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have not rewritten tests relating to key interfaces which would affect backward compatibility